Active Noise Control (ANC) systems are well known. They reduce unwanted noise in a room, such as in a car, by emitting a second noise with the same amplitude but inverted phase in that particular space. The original noise and the emitted noise cancel each other widely, which results in a lower sound pressure level.
Lower frequency noise is difficult to cancel by passive sound reduction means such as porous absorption material, since this should have a thickness of about a fourth of the wavelength to reduce, which is, at normal conditions for temperature and pressure, 85 cm at 100 Hz, but only 4 cm at 2000 Hz. Low frequencies are also very penetrating and travel long distances. On the other hand, the long wavelength of the lower frequencies is a benefit when using ANC: in a large area, like the head area of a passenger, the variation of the phase of the sound pressure level is small. By emitting the anti-sound to this area, the resulting noise in these low frequencies is greatly reduced. At high frequencies, on the other hand, the same local field comprises a high number of wave maxima and minima which cannot be compensated by interference sound waves of a single source, since the directions of sound propagation and anti-sound propagation is usually not coincident.
Several ANC systems for aircrafts are known in U.S. Pat. No. 5,310,137, which is hereby incorporated herein by this reference for all purposes, an ANC system for a helicopter is described for cancellation of high frequency noise. It comprises modified transmission beams with several mounting fixtures on which a plurality of force generating actuators and a plurality of sensors are mounted. Using this method, the structure borne noise can be cancelled right at the source. The problem is to calculate the anti-noise in a very short time.
At U.S. Pat. No. 4,819,182, which is hereby incorporated herein by this reference for all purposes, electro hydraulic actuators are mounted between the fuselage and the rack structure supporting the gearbox and the rotor of a helicopter to apply a vertical force to the fuselage that is reacted by an equal and opposite force on the rack.
The document U.S. Pat. No. 6,138,947, which is hereby incorporated herein by this reference for all purposes, describes an ANC system for a helicopter, where the input information is measured by a plurality of microphones or accelerometers arranged in the cabin above the heads of the passengers building a sub-system. For cancellation of the noise a plurality of actuators is arranged on each flange between the gear box and the cabin structure, generating the anti-noise.
Other ANC Systems described in U.S. Pat. No. 5,568,557, US 2016/0284337 and in U.S. Pat. No. 7,062,049, which are hereby incorporated herein by this reference for all purposes, use microphone and speakers, which are all mounted within the passenger cabin such as in the seats or in the trim, in a roof panel or to the rear shelf of a car for example.